1. Field of the Invention
The present invention relates to a control circuit for a circuit interrupter, and, more specifically, to a control circuit for selectively causing a circuit interrupter to operate in response to overcurrent in an alternating-current line in which the circuit interrupter is located. As used herein, "overcurrent" means current exceeding a threshold value.
2. Description of the Prior Art
Some prior art protective devices, such as fuses, circuit breakers, circuit switchers, and reclosers, may operate according to an inverse time-current relationship. Specifically, the higher the level of overcurrent in a line, the more quickly the protective device operates. Conversely, the lower the level of overcurrent in the line, the more slowly the protective device operates. Some prior art protective devices also respond to overcurrent in the line "instantaneously," that is, with minimum or no time delay after the inception of overcurrent in the line.
For example, the following publications are directed to arrangements involving instantaneous trip signal generation:
T. Keders et al, "A Current-Limiting Device for Service Voltages Up To 34.5 kV", pages 1-7, July 18, 1976, IEEE PES Summer Meeting, Paper No. A 76 436-6; PA1 C. Bottger, "The Application of I.sub.s -Limiters in Three Phase Systems", pages 1-7, 1967, Calor-Emag Company, Germany; PA1 Calor Emag Company, Leaflet No. 1197/6E, "Fault Levels Too High?", 2 pages, date unknown; PA1 M. C. Blythe, "Limiting Fault Currents Between Private and Public Networks", 4 pages, Oct. 5, 1973, reprinted from Electrical Review, Calor-Emag; PA1 N. Miyoshi et al, "Ultra-High Speed Protection Device--Fuji Ultrup Fuse", pages 47-51, Vol. 18, No. 1, Fuji Electric Review; PA1 P. J. Kroon, "The Development and Application of a 69-kV Fault Current Limiter", pages 237-244, Apr. 1, 1979, 7th IEEE/PES Transmission and Distribution Conference and Exposition; PA1 J. Wafer, "The Impact of Solid-State Technology on Molded Case Circuit Breakers", pages 605-611, Sept. 10, 1979, Vol. 1A-16, No. 5, September/October 1980, IEEE Transactions on Industry Applications; PA1 "Pyristor Pyrotechnic Systems", Carbone Ferraz, Inc. brochure; PA1 "Prototype Fault Current Limiter", EPRI Report, EL-1396, May 1980, Palo Alto, Calif.; PA1 U.S. Pat. No. 3,831,061 to Boyd; PA1 U.S. Pat. No. 4,442,472 to Pang; PA1 U.S. Pat. No. 4,017,766 to Vercellotti et al; PA1 U.S. Pat. No. 4,000,446 to Vandevier et al; PA1 U.S. Pat. No. 4,347,541 to Chen et al; PA1 U.S. Pat. No. 4,149,210 to Wilson; PA1 U.S. Pat. No. 3,290,556 to Graham et al; PA1 U.S. Pat. No. 3,317,791 to Price et al; PA1 British Pat. No. 1,436,861; PA1 U.S. Pat. No. 4,380,785 to Demeyer et al; and PA1 U.S. Pat. No. 4,393,431 to Gilker.
The Wafer article discusses the generation of various TCC (time-current characteristic) curves for circuit breakers and is operated in various trip modes including a long delay i.sup.2 t time characteristic, an instantaneous mode, and a short time delay mode. The Miyoshi et al article also uses i.sup.2 t as a criterion.
The remaining references listed in this category relate to various protection devices using di/dt as a criterion for operation.
For example, the EPRI report, EL-1396, and the Kroon article are directed to a fault current limiter that combines di/dt sensing in conjunction with time delay to prevent tripping on transients such as lightning or capacitor switching surges. The EPRI report, Section 7, at pages 7-4 and 7-8 and in FIG. 7-3, relates to a circuit having an RC timer that is initiated by the sensed current exceeding an I pickup current. A di/dt signal controls the RC timer so as to disable or reset the timer if the di/dt falls below a selected level. Similarly, the timer is reset if the current falls below the I pickup current. The report determines that the di/dt must exceed a specified level for a finite period of time (0.4 msec for the cases studied) to effectively filter out transients without seriously increasing the sensing time. The circuit in FIG. 7-3 describes the two transistors responsive to the di/dt signal and the I pickup signal as providing an AND gate.
The Blythe reference discusses an installation of a Calor Emag I.sub.s limiter which utilizes di/dt sensing. If the selected di/dt value is exceeded, the current limiter is permitted to operate if the current is between a lower limit I.sub.2 and an upper limit I.sub.1. The value of I.sub.2, the lower limit, is set so that the current limiter is not operative at the initiation of the fault when higher frequency currents might give it a false signal. The current level I.sub.1, the upper limit, is set to ensure an effective limitation of short circuit current.
The Carbone Ferraz brochure discusses various aspects of fault current protection devices such as di/dt sensing and, in a detailed description, "Current Limiting Function", starting on page 9, describes various time periods of operation such as the time TS required to reach the triggering current, the time TF required for analyzing the supplied signal and sending an electronic trip signal, the time TR for the mechanical response time, and the time TP for the fuse pre-arcing time.
The following patents are directed to current sensing and fault interrupting arrangements that include various combinations of short, long and instantaneous sensing and trip signal circuits, the various circuits providing diverse time current characteristics:
While the aforementioned arrangements are generally suitable for their intended purposes, it is an object of the present invention to provide a control circuit that operates according to an instantaneous relationship, the relationship being defined by the rate of change of the current exceeding a predetermined value for a predetermined time, and the instantaneous current at the expiration of the predetermined time being above a lower limit and below a higher limit.
It is another object to provide a control circuit that operates according to an instantaneous relationship wherein a trip signal is generated based on the rate of change of the current exceeding a predetermined value for a predetermined time interval.
Yet another object of the present invention is to provide a versatile control circuit that may be used to control the operation of or otherwise affect a wide variety of devices other than circuit interrupters.
Still another object to the present invention is to provide a control circuit for use with a current interrupter or other device and having both a time-current relationship and an instantaneous relationship that provides a desirable combination of minimum operating times and false signal immunity, matches the damage curves of equipment connected to a line, and coordinates with other protective devices in the line.